1. Field of the Invention
The invention relates to the use of radiation-curable coating compositions based on radiation-curing organopolysiloxanes with (meth)acrylic ester groups for the production of adhesive coatings which, for the purpose of photoinduced curing, comprise photoinitiators of the hydroxyalkylphenone type.
Adhesive coating compositions (release agents) are used to a wide extent to coat web materials to reduce their ability to adhere to adhesive surfaces.
Adhesive coating compositions are used, for example, to coat papers or films which are used as backings for self-adhesive labels. The labels, provided with a pressure-sensitive adhesive, do adhere to the coated surface to a sufficient extent to allow the label laminate to be handled. The adhesion of the adhesive labels to the backing must be sufficiently high that during label dispensing, e.g. to containers, the labels do not separate prematurely from their backing. On the other hand, however, the labels must be able to be peeled from the coated backing film without any substantial reduction of the adhesive tackiness. For this purpose the curing of the silicone release layer must be particularly effective, as otherwise silicone components may transfer to the adhesive surface and reduce tackiness. Further fields of applications of adhesive coating compositions are packaging materials which are used in particular for the packaging of sticky goods. Adhesive papers or films of this kind are used, for example, to pack foodstuffs or industrial products, such as e.g. bitumen.
A further application of adhesive coating compositions is in the production of self-stick closures, as for disposable diapers, for example. If the adhesiveness is too high, i.e., if the release force is too low, the diaper does not stay reliably closed. If the adhesiveness is too low and thus the release force too high, the closure can no longer be opened without destructive tearing of the diaper.
2. Description of the Related Art
The preparation of organosiloxanes with (meth)acrylate-modified organic groups attached to the siloxane unit via Si—O— and/or via Si—C— bonds is described in numerous patents. The patents and laid-open specifications below are given as representatives of the prior art and are herein incorporated by reference.
Organopolysiloxanes in which the (meth)acrylate-containing organic groups are attached to the polysiloxane framework via an Si—O—C— bond are described in DE-C-27 47 233 and DE-C-29 48 708.
Organopolysiloxanes where the acrylic ester-containing organic groups are attached to the polysiloxane framework via Si—C— bonds can be prepared, for example, by the hydrosilylation of a hydroxysiloxane with allyl glycidyl ether or another suitable epoxide having an olefinic double bond and, after the addition reaction, esterifying the epoxide with acrylic acid by opening the epoxide ring. This procedure is described in DE-C-38 20 294.
A further possibility of preparing (meth)acrylate-modified polysiloxanes with Si—C linkage of the modifying group(s) is the hydrosilylation of a hydroxysiloxane with an alcohol having an olefinic double bond, e.g., allyl alcohol, in the presence of a platinum catalyst and then to react the OH group of this alcohol with acrylic acid or with a mixture of acrylic acid and other saturated or unsaturated acids. This procedure is described, for example, in DE-C-38 10 140.
It is possible, furthermore, to attach in each case two or more (meth)acrylate groups per link to the silicone framework. In order to obtain very effective crosslinking—in other words, a very high number of reactive groups—while at the same time minimizing the density of modification on the siloxane framework it is desirable to attach more than one (meth)acrylate group per link. Processes of this kind are described, for example, in U.S. Pat. No. 6,211,322.
Examples of these organosilicone compounds are available, for example, from Goldschmidt/Germany under the product name TEGO® RC, from Shin Etsu/Japan under the name X-8010 and from Rhodia/France under the name UV 900, UV 911 and UV 970.
In order to produce said coatings the normal approach is to apply a mixture of two or more of said organosilicon compounds (and one or more photoinitiators) to web materials made of plastic, metals or paper and to pass the resulting material in web form from roll to roll, at high line speeds of several hundred meters per minute, through an UV unit to cure them.
In contrast to said organosilicon compounds various purely organic compounds, i.e. compounds not having silyl or siloxane units, are known which contain double bonds capable of free-radical polymerization and which after the addition of known photoinitiators cure by UV radiation. These coating compositions are used, for example, for printing inks and for preparing film-forming binders or for the coating of surfaces of plastic, of paper, of wood, and of metal.
A feature common to all said organosilicon compounds and organic compounds is that after the addition of photoinitiators, they cure within a very short time by UV irradiation in a free-radical polymerization. An overview of suitable photoinitiators is given in “J. P. Fouassier, Polymerization photoinitiators: Excited state process and kinetic aspects, Progress in Organic Coating, 18 (1990) 229-252”, in “J. P. Fouassier, Photochemical reactivity of UV radical photoinitiators of polymerization: A general discussion, Recent Res. Devel. Photochem. & Photobiol., 4(2000):51-74”, in “D. Ruhlmann et al, Relations structure-proprietés dans les photoamorceurs de polymérisation-2. Dérivés de Phenyl Acetophenone, Eur. Polym. J. Vol. 28, No. 3, pp. 287-292, 1992” und “K. K. Dietliker, Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, Volume 3, Sita Technology Ltd, UK”. DE-A-27 22 264 describes hydroxyalkylphenones and their use as photoinitiators.
These literature references cite photoinitiators, including photoinitiators of the hydroxyalkylphenone type, and evaluate their reactivity in the stated organic compounds. The activity of photoinitiators is dependant on a multiplicity of factors, which are substantially also dependent on the photoreactive group and on the concentration of said group in the photoinitiator molecule—that is, on the ratio of the molar weight of the reactive group to the overall molar weight. The molar weight of the free-radical cleavage products and hence of the photoinitiator molecule itself is also of importance for the activity. These relationships are prior art, particularly for photoinitiators of the hydroxyalkylphenone type.
Photoinitiators must also be soluble in the coating composition in order to achieve a sufficient activity in UV curing. As a result of the very hydrophobic matrix of the stated organosilicon compounds, in contrast to the organic compounds, it is not possible to use a large number of the known photoinitiators, since they cannot be dissolved compatibly in the silicone matrix. Particularly suitable photoinitiators for the stated organosilicon compounds are, therefore, those which are liquid or easily meltable and which compatible blend to the silicone matrix.
In order to improve the solubility with the stated organosilicon compounds, WO-A-97/49768, U.S. Pat. No. 5,776,658, U.S. Pat. No. 4,391,963 and EP-B-0 088 842, for example, propose photoinitiators modified with silyl radicals, including in particular polymer silyl radicals. The photoinitiators are, for example, of the hydroxyketone, aminoketone, benzoin ether, benzophenone or thioxanthone type. Patents U.S. Pat. No. 4,356,265, U.S. Pat. No. 4,534,838 and EP-B-0 162 572 as well describe a very wide variety of photoinitiator structures provided with organopolysiloxane radicals. These compounds are derived, for example, from dialkoxyacetophenone and have an increased silicone solubility. U.S. Pat. No. 4,507,187 discloses diketo photoiniators with silyl groups as photoinitiators which are readily soluble in silicone polymers. Polymeric photoinitiators containing siloxane radicals are described in U.S. Pat. No. 4,587,276. A. Kolar et al in J.M.S. Pure Appl. Chem. A31(3) (1994), 305-318 on reactive silyl-derivatized hydroxyalkylphenone photoinitiators. EP-A-1 072 326 describes siloxane-modified photoinitiators with surface-active properties, which again are derivatives of hydroxyalkylphenone.
The literature references cited deal with solving the problem of the solubility of photoinitiators in the stated organosilicon compounds. Solubility is achieved by attachment of partly polymeric siloxane radicals to the photoreactive group. This solution has the drawback that the concentration of the photoreactive group is reduced, as a result of the high molar weight of the silyl or siloxane radical, to the extent that the solubility with the stated organosilicon compounds rises. In order to compensate the reduced concentration of the photoreactive group, it is necessary to admix increased concentrations of such photoinitiators to the stated organosilicon compounds. Moreover, the process steps needed for the attachment of the silyl or siloxane radicals constitutes an additional process step, which adds to the costs.
There is therefore a need to find, for the stated organosilicon compounds, a readily available, readily miscible, and compatible photoinitiator which is nevertheless equipped with a high concentration of photoreactive groups.